Ceramic permanent magnet

ABSTRACT

MAGNETOPLUMBITES HAVING THE FORMULA PHO.NFE203, WHEREIN N IS FROM ABOUT 3 TO ABOUT 6.5, HAVING DENSITES OF NOT LESS THAN 85% OF THE THEORETICAL MAXIMUM, HAVING AVERAGE CRYSTALLITE SIZES OF LESS THAN ABOUT 2.0 MICRONS, HAVING AT LEAST 90% OF THEIR CRYSTALLITES LESS THAN 2.5 MICRONS IN DIAMETER AND CRYSTALLITE ORIENTATIONS OF NOT LESS THAN ABOUT 70% YIELD PERMANENT CERAMIC MAGNETS HAVING BOTH HIGH COERCIVE FORCES AND REMANENCES.

Aug. 20, 1914 c. M. SCHLAUDT ET AL 3,830,743

CERAMIC PERMANENT MAGNET Filed Sept. 27, 1971 0 s o D a m .D R E o w m$38 53. "w o a l 0 o o O 4 2 O o 2 W FIELD FIG.

FIG.

FIG. 4

FIG. 3

us. or. 252-6163 United States Patent 3,830,743- CERAMIC PERMANENTMAGNET Charles M. Schlaudt, Berkeley, Ronald L. Clendenen,

. Orinda, and Eugene E. Olson, Oakland, Calif., assignors to Shell OilCompany, New York, N.Y.

' Filed Sept. 27, 1971, Ser. No. 183,895

Int. Cl. H01f 1/02 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THEINVENTION Field of the Invention The Prior Art Ceramic permanent magnetshave a wide spectrum of application, especially in electrical devicessuch as loudspeakers, motors, and the like. In many applications it isdesired to use a magnet having both a high remanence and a high coerciveforce. Such a magnet is resistant to demagnetization and is magneticallystrong.

With conventional ceramic magnets it has been difficult to achievesimultaneously relatively high values for these two properties. Thisproblem is inherent in conventional ceramic magnets and their method ofproduction. To yield an ideal magnet of maximum remanence and coerciveforce a ceramic material must have four properties: (a) it must be fullyferritized, that is, there must have been full reaction between the leadand iron oxides to form a chemically bound mixed oxide (Pb0-6Fe O' sincethe individual oxides are not magnetizable; (b) the mixed oxide must bein the form of small crystallites, preferably sized equal to or justabove the size of a magnetic domain, i.e., 0.1 to 1.0 microns; (c) thecrystallites must all be oriented, that is, lined up so that their axesof easy magnetization are all parallel; and (d) the material must be asnear to the theoretical maximum density as possible. With conventionalpreparation methods, such as described by Haag in Annual Report March1969-70 concerning Ofiice of Naval Research Contract N000l4-68-C-0364 orby 'Sixtus et al. in 27 Journal of Applied Physics 1051 (September,1956), those conditions which lead to small crystallite size workagainst obtaining a full orientation and high density and vice versa.

I It is known that metal ferrites having the chemical formula PbO-nFe- Owherein n has a value of about 6-,,say from 3 to 6.5, form permanentmagnets. These particular ferrite materials are also calledmagnetoplumbites. Table I is a listing of the magnetic properties(remanences and coercive forces) obtained with a variety of lead ferrite3,830,743 Patented Aug. 20, 1974 magnets produced heretofore forcomparison with tlie magnets of this invention.

TABLE I [Magnetic properties of prior art lead ferrite magnets] NormalIntrinsic STATEMENT OF THE INVENTION It has now been found that a leadceramic permanent magnet having both a high coercive force and a highremanence, that is, an intrinsic coercive force of not less than 2000oersteds and a remanence of not less than 2800 gauss, is prepared from amagnetoplumbite of the formula PbO-nFe O wherein n has a value of from 3to 6.5, wherein the density is not less than of the theoretical maximum,the average crystallite size is not greater than 2.0 microns and acrystallite orientation is not less than 70%.

This invention will be further described below with reference to thedrawing wherein FIGS. 1 and 2 are magnet hysteresis loops and FIGS. 3and 4 are scanning electron photomicrographs, respectively, of magnetsin accordance and not in accordance with this invention.

Definition of Terms As this invention is directed to magnets withimproved properties, for the sake of completeness it is desirable atthis point to set out precisely what these properties are and theirimportance. This will be done with reference to FIG. 1 of the drawing.FIG. 1 is a graphic representation of the amount of magnetic fluxinduced in a permanent magnet material when it is exposed to a varyingmagnetizing-demagnetizing field. It is in the form of two quadrants ofan intrinsic hysteresis loop. FIG. 1 also contains a portion of thenormal hysteresis loop for the same material.

A sample of unmagnetized material by definition has no induced flux at Ofield and thus is at point 0 in FIG. 1. As an increasing magnetizingexternal field (+H) is applied, the flux induced in the sample followsin line OW and reaches a constant value referred to as the saturationmagnetization (B As the positive field is reduced, the flux follows theline WX. The flux remaining when the field has been reduced to zero istermed the remanence (B,). As an increasing demagnetizing external fieldis applied (H), the induced flux followsthe line XY. The demagnetizingfield required to decrease the induced flux to zero is referred to asthe intrinsic coercive force (H of the material. Alternatively, as thedemagnetizing field is increased the normal induced flux will decreaseto zero along line XZ. The point at which the normal induction reacheszero is termed the normal coercive force I-I As can thus be seen, theresistance to demagnetization which a material possesses is indicated bythe intrinsic coercive force, H The strength of a magnet is given inpart by the value of the remanence. The product of induction andi'external' field as given by the normal demagneti .zation curve (line XZ)reaches a maximurnat some B and H. The value of this product is used asa figure of merit for permanent magnet materials and is referred .to asBH or the energy product. -It. may be seen from FIG. 1 that theremanence must be equalto or less than the saturation magnetization. Theintrinsic coercive force must be greater than or equal to the normalcoercive forcelt may also be seen that the normal coercive force cannotbe larger than the remanence or else the intrinsic induction wouldincrease even though a demagnetizin-g external'field is being applied--aphysical impossibility.

DETAILED DESCRIPTION OF THE INVENTION Chemical Composition of theMagnets wa e A1203, ZIO B203, Bi203, Cal- SISO4, CaSO and the V like.

Magnetic Properties of the Magnets The most striking features of themagnets of this invention and their unusual combination of highremanences and coercive forces.

The ceramic magnets of this invention are generally characterized ashaving remanences of greater than 2800 gauss as well as intrinsiccoercive forces of greater than 2000 oersteds, and are more particularlycharacterized as having remanences of from 3000 gauss to about 4000gauss and intrinsic coercive forces greater than 2200, especially 2300to 4500 oersteds.

Preferred magnets of this invention are further characterized as havingremanences which are substantially similar to their saturationmagnetizations. This feature is illustrated by FIG. 2, a graphillustrating the magnetic properties of a preferred lead ferrite inaccord with this invention. As an increasing magnetic field is appliedto a sample of unmagnetized lead ferrite of this invention its magneticflux increases along line ab, eventually reaching a maximum (saturation)value B, at (b). Increasing the field further will not increase the fluxbeyond B As the field is reduced to zero, the flux follows line he andreaches its remanence value B As the figure illustrates, the remanencehas essentially the same value as the saturation magnetization.

The ratio of remanence to saturation magnetization is one measure of amagnets extent of crystallite orientation since crystallographicorientation is related to magnetic orientation. An ideal singlecrystallite has a remanence along its axis of easy magnetizationequivalent to its saturation magnetization. A plurality of crystallites,if perfectly oriented, would exhibit a r Bf I as well. Thus, the extentof crystallite orientation can be determined'by'the ratio B /B The highstrength magnets of this invention find application in-a variety ofareas, for example in electric m0- tors, in "loudspeakers, and inholding applications.

Physical Properties of the Magnets in the range of from 0.5 to 2.0.microns and most preferably from 0.6 to 15 microns. Not only are thegrains o t se ag ets .small i siz t ey arealsdunif rtni size. Suitablyat least about 90% of the grains have diameters of less than 2.5 micronsand preferably at least about 90% have diameters of less than 2.0microns.

The unique crystal structure of the magnets of this invention is.clearly' shown by a scanning electronmicroscope. FIGS. 3: and 4areequivalent scanning electron photorni'crographs respectively of .a';magnet 0fthis invention and an excellentquality'conventiorial fer'ritemagnet (sold under the trade name Steward 504) showing the uniform finegrainsiz'e of thepresentmagmtsand the irregular grain size ofconventional materials.'The grains of the magnets of this invention arepreferably highly oriented. Preferably, at least 70% of the crystallitesare oriented, with orientations (as shown by the ratio of remanence oversaturation magnetizationyof not less than being most preferred.

Preparation v of the Magnets Ferrite ceramic permanent magnets in"accord with this invention are prepared by a 'hot forgin'g process whenthis process is carried outunder "certain controlled conditions. Thisprocess comprises the steps. of;

(a) Preparing solid particles comprising an intimate agglomerate ofsuitable proportionsof less than 0.1 micron grains of ferric oxide andlead oxide;

(b) Heating these particles for'up to 24 hours at a temperature in therange of from 700? C. to 10009C. to effect at least a partial chemicalreaction betweenthe iron and lead oxides (ferritization) andproducelessthan 0.5 micron ferrite crystallites; I

(c) Heating the atleast partially ferritized particles for up to 2 hoursat 750 C. to 1000 C. (preferably while applying up to 30,000 p.s.i.pressure) toelfect at least a partial sintering together of theparticles into a solid body; H

(d) Heating at 700 C. to 1050 C. and pressing at up to 30,000 psi. (hotforging) the resulting sintered body to density it to at least,95% oftheoretical maximum, to completely ferritize it, and to deform it andthus align its magnetic crystallites; and 1 (e) Magnetizing theresulting ferrite compact by con ventional means to give a permanentmagnet.

In the first step of this process small solid particles are preparedwhich consist essentially of :ferric oxide, and lead oxide, and anydesired additives. These components should be present in the sameproportions as desired in the finished magnets.

The particles formed in this step are themselves agglomerates ofparticles of the iron oxide and lead oxide; With any of thesecompositions, it is very desirable that these agglomerates be small andis essential that the" particles which make up the :agglomerates be'very small in size. The agglomerates must be made'up o'fparticles' offerric oxide and lead oxide which are 'less than 0.1 microns indiameter. Preferably the individual oxide par ticles which make up the.agglomerates are less'thanfiJO-Z micron in diameter. r Suitableagglomerated particles are produced by sev eral techniquesJn one method,for example, they-are'pree pared bycoprecipitating a mixtureof-decomposable compounds of the metals and then thermally decomposingthe precipitate. Other techniques include, for example, spray drying orspray roasting a mixed salt solution. These methods all lead to veryintimately mixed agglomerates of lless than 0.1 micron particles offerric oxide-and lead 0x1 e. 7

Using the coprecipitation technique, a solution of correct molar ratio,most conveniently in water, of soluble ferric and lead and optionallyadditive salts is first pre-' pared. Examples of suitable salts includeferric nitrate,

acetate, chlorate, formate, and oxalate; and leadfnitrate, nitrite,citrate, and acetate.

The solution of salts is then treatedwith a precipitating agent whichgives a thermally decomposable precipitate. Examples of suitableprecipitating agents are hydroxyl ion, carbonate,ion,. andthe like.Preferred precipitating agents are hydroxide ion and carbonate ion inamount of from about one. toabout ten times the stoichiometric amountrequired for precipitation of all the metal ions present.

The mixed precipitate is separated and thermally decomposed in anoxygen-containing atmosphere to give the agglomerated particles of theoxides. Generally, an ex- .posure of from about 2 to 36 hours attemperatures in the range of from about 400 C. to about 700 C. isadequate to carry out the decomposition. Longer times and highertemperatures may be required with very difficult to decompose salts.

" Using the spray drying technique, first a solution is preparedcontaining a decomposable ferric salt and a decomposable salt of lead inthe desired 3:1 to 6.5 :1 molar ratio. Suitable salts include nitrates,carbonates, acetates, chlorides and like materials which decompose whenheated in the presence of oxygen. Any additives should also be presentin this solution to ensure their ultimate intimate admixture with theprincipal metal oxides. The solution is atomized into a chambermaintained at an inlet drying temperature in the range of from about 100C. to about 600C., preferably from 200 C. to 500 C., to form small, dryparticles of mixed decomposable salt. These particles are then thermallydecomposed in an oxygencontaining atmosphere. This decomposition step issimilar to that described with the coprecipitation method of formingparticles and requires similar conditions.

Using the spray roasting technique, a solution of decomposable salts isprepared and atomized into a chambet or fluidized bed having anoxygen-containing atmosphere heated to a temperature in the range offrom 500 C. to 1000 C. In one step the particles of mixed decomposablesalts are formed and thermally decomposed to mixed oxides.

Each of these techniques gives fine agglomerates having internalparticles of ferric and lead oxides which have -diameters of not greaterthan about 0.1 microns.

The particulate solids produced in the first step are agglomerates ofessentially distinct grains of ferric oxide and grains of lead oxide. Inthis step of the process, these agglomerates are heated to a temperatureof 700 C. to 1000 C. for up to 24 hours to cause these separate oxidegrains to chemically react and form small crystallites of lead ferrite.This heating step is known as ferritizing. It is essential that thetemperature and period of this heating be closely'controlled. Thetemperature must be maintained high enough to cause the metal oxides toreact with one another but must not be substantially above the reactiontemperature or else undesired particle grain growth will occur. It isnot necessary that this ferritization be carried to completion. It ispreferred to obtain full ferritizati on partially by heating in thisstep and partially by heating with pressure in the next two productionsteps (forming and forging). When pressure is applied with heat in theforming and forging steps which follow, full ferritization is achievedat lower temperatures and thus with far less chance of undesired graingrowth. Temperatures selected in the range of from 750 C. to 900 C. arepreferred for full or partial ferritization as are heating periods offrom 0.5 to 8 hours, it being understood that the higher temperaturesrequire shorter times while lower temperatures require longer times.

Examples of suitable ferritizing conditions are: about 24 hours at 600C., about 4 hours at 800 C., and about 0.5 hour at 900 C.

The metal ferrite powder next is formed into a compact solid mass eitherby the application of heat (sintering) or preferably by the applicationof heat and pressure. This step is required since an essentially solidbody 6 I must be employed in the hot forging step which follows toprepare the actual magnetiza'ble ceramic material.

In this sintering step the emphasis is on the relatively quick heatingwhich permits sintering while minimizing grain growth. Generally,heatings of up to about 2 hours at 750 C. to 1000 C. give a goodsinteredproduct, more specifically, 0.1 to 2.0 hours at 800 C. to 900. C. arepreferred. Examples of suitable sintering conditions 316% about 1.5hours at 800 C. and about 0.5 hour at In a preferred method ofoperation, heat and pressure are both employed to effect compaction andsintering. The use of pressure permits lower temperatures and/or shortertimes to be employed and thus further limits grain growth. Very suitablehot pressing conditions are in the range of from 700 C. to 1000 C. andpreferably 800 C. to 950 C. and from 1000 to 30,000 p.s.i. Use ofconditions in this range, enable suitable compaction to be effected in atotal heating cycle of about 15 minutes or less, preferably from 1.0 to10 minutes. Typical pres sure sintering conditions are:

10 minutes at 850 C. and 15,000 p.s.i.,

10 minutes at 950 C. and 5,000 p.s.i.,

3 minutes at 950 C. and 20,000 p.s.i., and 1 minute at 1000 C. and10,000 p.s.i.

The sintering or hot pressing may be carried out in an oxygen containingenvironment (air) in an inert environment (nitrogen) or in a vacuum.

The nature of the product of the sintering (or preferably hot pressing)is critical to the success of this process. To ultimately yield thedesired high quality magnets, it is essential that the product of thisstep be made up of uniform, less than 1 micron diameter crystallites.When heat and pressure are applied in this step the products are moreparticularly characterized as being solids, having densities of from 80to 100% of the theoretical maximum. When heat alone is applied theproducts are solids of lower density, generally 40 to 80% of thetheoretical maximum. These materials must have this lower densitybecause more severe heatings necessary to achieve higher densities alsogive undesired amounts of grain growth. In this case, full density inaddition to crystallite orientation is achieved in the following hotforging step. In both cases the solid products are made up ofcrystallites having an average diameter preferably less than 0.7 micron,especially from 0.3 to 0.7 micron, and having not more than 10% of theirdiameters greater than 1 micron. When such a product is oriented and ifnecessary densified in the hot forging step, a superior magnet results.Without further treatment, this product might be useful in low qualitycrude magnet applications, but might not be suitable as a desirable highquality magnet.

The fine grain solid ferrite body formed in the sintering step has theproperty of being ductile when heated to a temperature approximating itsforming temperature. This property is utilized in the hot forging stepto effect the full densification and orientation of ferrite grainsessential to the production of an anisotropic permanent magnet. The hotforging is carried out by heating the ferrite body and applying apressure to it in a manner which deforms it. As in the sintering step,the emphasis is on a rapid treatment with a limited exposure to hightemperatures to minimize grain growth. Conditions similar to the hotpressing conditions optionally used to form the solid ferrite body maybe used for hot forging. Temperatures of from 700 C. to 1050 C.,preferably 750 C. to 1000 C. and pressures of from 1000 to 20,000p.s.i., preferably 3000 to 20,000 p.s.i., and times of up to about 0.5hour, preferably up to about 0.2 hour are useful, with times of from 0.1to 5 minutes being preferred.

The temperature and pressure are most favorably controlled to give astrain rate of from about 1%/min. to about 500% /min. The preferredtemperature and pressure conditions noted above fall into this area.

"wherein L is the size of' the body along the itforging axis'afterforging, and L5 is thesize before forging. To achieve thei s amedegree at orientation, non-dense bodies will require different degreesofforging than dense bodies for, in the former, a certain amount offorging will be taken up in the densification process. After thematerial has been forged to essentially theoretical density, additionalforging will produce bulk flow of the material resulting in orientation.The deformation of a body by forging can be approximated by the equationo pr o r 1 o (P pr L wherein p,- is the density relative to theoretical.The first term in parentheses represents the contribution ofdensification to forging, and the second term represents thecontribution of mass flow. For any given density, the most suitablevalues for fOrging, represented by will range from 0.1 to 0.9 withvalues between 0.2 and EXAMPLE I Preparation of PbO-'5.8 Fe O magnet A.Starting Material Preparation: 7001 grams of Fe(NO -9H O and 498.75grams at Pb(NO were dissolved in 13 gallons of water. This solution wasdried in a Niro Portable Spray Drier. The inlet temperature was 430 C.and outlet temperature was 160 C. The material from the spray drier wasplaced in ceramic crucibles and heated to 600 C. for 15 /2 hours toremove the nitrates.

B. Ferritization: To ferritize the material, the crucibles containingthe powder were placed in a furnace at 800 C. for 4 hours in air. Thecalcined material was ground/ crushed to pass an 80 mesh sieve. Thismaterial consisted of less than 0.5 micron particles of lead ferrite.

C. Hot Pressing: The ferrite powder was loaded into a graphite die andthen heated to 900 C. in a graphite heating element furnace. Appliedpressure was about 4000 p.s.i. The furnace atmosphere pressure rangedfrom 35 to about 350 microns during the hot pressing operation. Thesample was held at temperature for about 15 minutes to effect sintering.The sample was cooled in the furnace, removed, and cored into smallerspecimens which were later to be hot forged. Property measurements onthe hot pressed sample gavethe following results:

Density=5.38 gm./cc.' B =3220 gauss B,=2640 gauss H=2230 oersteds H-3510 oersteds BH =L5 X gauss-oersteds The materialwas' a solid havingan average grain size of about 0.2 microns. V

D. Hot Forging: Samples cored from the hot pressed material were forgedto different degrees. Strain rates on all of the forgings were keptconstant at 10% /min. The forging temperature was 1000 C. The forgingatmosphere wasair. The sampleslwere placedin a cold furnace, the u n cth b in jh atp vtQth 'j Q gi p atl s 1119 minutes. The samples wereallowed to 'equilibrate for 15 minute s"at temperature, then i sufiicie'nt load, \vas 'ap.- plied to' defo rm the specimen's'at the.required rate, {The samplesyvere' cooled'in airiand removed frorn'tli'efurf nace. Their properties;were as follow's:

o percent; '50. 5 61.4 71; 6

w I I w Density, g./ee I 5.41 --5. 42' 5. 44 B., gauss 3, 800 3,900 3,85r, gauss- 3, 750 3, 900 3,83 HG, oersteds I 2, 700- :2, 650 2,580 Hoersteds 2, 800 2, 700 2, 620 Bu aus -Deleted 4 1o a; 7x10 316x10 Gramsize, microns v 1. 1 Less than 10% larger than v 1 1% i1 Theoreticalmaximum density 5.57 grams/cc. EXAMPLE 11 Preparation of PbO-6Fe O' bycoprec'ipitation A. Starting Material Preparation: 87.9: grams, of Pb(NO(which included 3.8 grams excess to compensate for the solubility atPb(OH) in the water o'fImixing and washing) and 1231.3 grams of Fe(NO-9H 'Q were dissolved in 4 liters of distilled water. This solution wasprecipitated to pH 9 with concentrated NH OII solution. The solution wasallowed to stand overnight and the precipitate was filtered and washedwith 24 liters of.dis

tilled water. The filter cakewas dried at 50 ,CQOverI- night and groundto mesh. j

B. Ferritization: The precipitated materiahafter reaction at 800 C. for4 hours, was examined and found. to be comparable to spray diredmaterial, both chemically and physically. r j

EXAMPLE III Using the general operating procedures of Example I, butvarying the conditions within the preferred ranges of this invention, avariety of magnets were prepared. These magnets had the followingproperties:

Lead Ferrite Production Fe(No -9H O and Pb (NO in the molar ratio of 12.to 1 are dissolved in water. This solutionis atomized and passed througha tube heated to about800 C. It has a residence time in the tube ofabout .2-3 minutes. The product of this spray roasting treatment is afine powder of mixed oxide. The powder particles are agglomerates ofless than 0.02 micron grains of oxides. The powder is maintained atabout 800 C. for about 4 hoursto permit the two metal oxides to react(ferritize). This powder is placed in a die and hot pressed at 10,000p.s.i. and 800 ,C. for 5 minutes to give a solid compact having adensity of about 80% of theoretical maximum. This body would have grainstructure wherein at least of the crystals have diameters of from 0.05to 1 microns. This body of material is placed between two platens andhot forged at 25,000 p.s.i. and 850C. until a I 9 of about 0.6 isachieved. This product would be a highly oriented ferrite, suitable forpreparing excellent magnets.

Ferrite Production Using Sintering Instead of Hot Pressing An experimentsimilar to Example I is carried out. Steps A and B are repeated. Theferrite powder of step B is formed into a pellet and maintained at 900C. for 0.5 hour to effect sintering. The resulting product would have adensity about 60% of the theoretical maximum. It would have a crystalsize distribution similar to that observed in Example I. When hot forgedin accord with Example I it would yield dense oriented products, similarto those obtained in Example I.

Comparative Experiments A series of comparative experiments wereconducted to demonstrate certain critical features of the presentinvention.

A. Orientation Using a Cold Orienting Process: A sample of the PhD-5.8Fe O powder prepared in accord with Example III was cold oriented in aconventional manner using a 4000 oersted magnetic field. This productwas sintered with pressure and its magnetic properties were measured. Ithad a remanence of 3100 gauss and an intrinsic coercive force of 2100oersteds.

B. Use of Too Severe Ferritizing Conditions: A series of samples wereprepared in accord with Example I, the temperatures, times and pressuresemployed were varied. The following ferritizing conditions were found togive poor final products.

A sample of PbO-Fe O was ferritized for 32 hours at 800 C. After hotpressing at 950 C. and 5000 p.s.i. in accord with Example I, the samplehad a low intrinsic coercive force (1100 oersteds) showing excessivegrain growth.

C. Use of Too Severe Hot Forging Conditions: A PbO-6Fe O sample wasprepared in accordance with Example 1. The hot forging conditions werevaried. The following conditions were found to be too severe. When the10 sample was forged at 900 C. and 4000 p.s.i. until a 93% deformation(L-L,,/L

was achieved, the resulting product showed inferior properties. It had aremanence of 3460 gauss and normal and intrinsic coercive forces of 1580oersteds each.

We claim as our invention:

1. A permanent ceramic magnet having an intrinsic coercive force of notless than 2000 oersteds and a remanence of not less than 2800 gauss andconsisting of a magnetoplumbite of the formula PbO-nFe O wherein n has avalue of from 3 to 6.5; and having a density of not less than 85% of thetheoretical maximum, an average grain size of less than 2.0 microns, notless than 90% of its grains less than 2.5 microns in diameter and acrystallite orientation of not less than 2. The magnet in accordancewith claim 1 wherein the coercive force is not less than 2200 oerstedsand the remanence is in the range of from 3000 gauss to about 4000gauss.

3. The magnet in accordance with claim 2 wherein n has a value of from4.5 to 6.5.

4. The magnet in accordance with claim 3 wherein the density is not lessthan of the theoretical maximum, the average grain size is from 0.6 to1.5 microns and at least of its grains are of diameters of less than 2.0microns.

References Cited UNITED STATES PATENTS 3,189,550 6/ 1965 Malinofsky25262.62 3,337,461 8/ 1967 Cochardt 25262.63 3,576,745 4/ 1971 Tokar,Ir. 25262.63

EDWARD J. MEROS, Primary Examiner I. COOPER, Assistant Examiner U.S. Cl.X.R. 423-594

